Ophthalmic optics may be cast by curing a polymerizable resin formulation between either a pair of molds spaced apart by a gasket, or between an optical preform and a front mold provided to form the front curvature and the add portion of the optic. Several methods for casting spherical or aspheric multifocal or progressive optics utilizing optical preforms have been disclosed. For example, U.S. Pat. No. 4,623,496, to Verhoeven discloses a method for casting multifocal ophthalmic optics from a preform and a mold without utilizing a gasket. U.S. Pat. Nos. 4,474,355 and 4,190,621 to Greshes disclose a method for casting ophthalmic optics using an optical preform and a front mold. In Greshes an edge fixture provides support to the preform and the mold and maintains the correct spacing therebetween.
Co-pending application Ser. No. 07/594,136, now U.S. Pat. No. 5,178,800, filed by the present inventor describes a method for casting single vision or multifocal ophthalmic optics that may be spherical or aspheric in geometry, and which may incorporate bifocal, trifocal or progressive addition zones. This method utilizes an optical preform that may be spherical or aspheric, and incorporates the distance prescription, including any cylinder that may be needed. The method also utilizes a front mold that has a curvature with a fixed relationship to the convex curvature of the preform which incorporates the add portion of the prescription, as well as a resin formulation having a specified viscosity which is placed in the space between the mold and the preform. Other known methods for forming an ophthalmic optic from an optical preform include those disclosed by Staehle, U.S. Pat. No. 2,339,433, Caulkins, U.S. Pat. No. 3,946,982, and Squires, Patent No. AU-A-80556/87.
For all of these known methods for forming ophthalmic quality optics from an optical preform to be successful, a secure and durable bond must be created between the preform and the cured resin layer formed between the preform and the mold. A criterion of good bonding in ophthalmic optics which has been adopted by the American National Standards Committee (ANSI) requires that the surface layer pass the "cross hatch test". In the cross hatch test, a series of deep parallel scratches are cut into the surface of the optic, followed by a second series of parallel scratches orthogonal to the first series, to form a grid pattern. Adhesive tape is securely bonded to the surface of the grid pattern and subsequently removed from the surface by pulling on the tape in a direction perpendicular to the optic surface. The tape and the optic surface are then examined for any evidence of delamination of the surface cast material. For the optic to pass the razor blade test, no delamination should be visible to the naked eye.
Bonding of the resin to the preform may be enhanced by optimizing the formulation. To accomplish this optimization, the surface energy of the preform and the resin layer should be as similar as possible to ensure excellent wetting of the preform by the resin. The resin formulation should include monomers that can readily diffuse into and permeate the surface of the preform. In practice this is achieved by ensuring that the preform and the resin formulation have at least one ingredient (monomer) in common, preferably the predominant one found in the preform. For example, if the optical preform is formed from a resin formulation based on bis diallyl carbonate (available commercially from PPG, tradename CR-39), the surface layer should be cast from a resin formulation containing CR-39 at a minimum level.
Another method for enhancing the bonding between the preform and the surface layer involves the surface treatment or surface modification of the preform. Several methods of surface modification have been developed for this purpose, and will be discussed in detail below.
Known surface treatment methods involve a chemical treatment of the preform surface, a physical treatment, or a combination of the two. Preferably, the chemical method involves chemical attachment of polymerizable silanes to the surface of the preform. Known physical treatments involve the development of a roughened surface on the preform surface which increases the area of contact between the resin and the preform surface, thus enhancing the microscopic flow of resin across the preform surface during the curing process to ensure complete coverage and uniformity of cure over the whole surface.